This application claims priority to German Patent Application No. 101 35 974.8, filed Jul. 24, 2001, which application is incorporated by reference herein.
This invention relates to a method for the production of a sealing element with a backing and a honeycomb structure attached to this backing in which the cellular arrangement of the honeycomb structure is formed by individual, profiled layers made of a thin, foil-like, metallic material, and in which the layers are bonded to each other and the honeycomb structure is bonded to the backing
Sealing elements of the type described are known in the state-of-the-art, for example on gas turbines or on thermal turbomachinery. The sealing elements are used in these applications as so-called abradable seal segments. The honeycomb structure is usually bonded to the backing by means of a special brazing and/or diffusion process. The backing itself is a casting or a sheet-metal plate, for example in the form of a ring or a ring segment.
The initially cited abradable seals serve to improve the performance and/or efficiency of the thermal turbomachine or the gas turbine by sealing the gap between a stationary and a rotary component, for example on blades or shafts, or by keeping such a gap as small as possible. Such honeycomb-type sealing structures are used, in particular, in the hot turbine section. They consist of thin metal-foil ribbons which are arranged to form honeycomb structures. The metal foils are cut-to-width and formed to create flat plateaus, these plateaus being parallel with each other and protruding down- or upwards. The known honeycomb structure is obtained by arranging various such metal-foil ribbons one on the other or one behind the other, respectively.
The honeycomb structure or cellular structure thus formed is used as abradable seal. Its configuration ensures that it exerts only minor resistance to the fast rotating components rubbing against it, thus preventing the rotating components, such as blades, from damage.
In the sealing elements known in the state-of-the-art, bonding of the individual layers to each other and bonding of the honeycomb structure to the backing is achieved by means of a brazing process. For this process, conventional brazing filler materials, such as alloys with a high nickel or cobalt content, are used. Since these nickel or cobalt alloys have high melting points, alloy elements which lower the melting point, such as boron, are added, thereby providing a suitable brazing filler material. Another variant is the provision of brazing filler materials in the form of eutectic alloy with an intrinsically low melting point. Here, the eutectic or near-eutectic compositions known in the nickel-silicon system are to be named. The brazing filler materials according to the state-of-the-art have a solidus temperature in the range of 800 to 1300xc2x0 C.
The elements lowering the melting point, such as boron or silicon, are detrimental in many respects. While the brazing filler material is in the molten state, these elements diffuse into the material of the honeycomb structure and of the backing forming intermetallic compounds, such as borides and silicides, for example of the type CrxBy, with the alloy elements of these materials. Normally, these intermetallic compounds are found at the grain boundaries of the base metals or at the grain boundaries of the re-solidified brazing filler material. They cause embrittlement of the base metals and of the brazing filler material and, hence, of the entire assembly of the sealing element.
A further disadvantage lies in the fact, that several of the intermetallic compounds formed, such as chrome-borides (CrxBy), are susceptible to corrosion.
Since the sealing elements according to the present invention are exposed to high operating temperatures by hot process media, they must be resistant to hot-gas attack and, hence, to high-temperature oxidation, high-temperature sulfidation and/or high-temperature carbonization. In the sealing elements according to the state-of-the-art, this is achieved by using high chrome-alloyed nickel-base alloys for the material of the honeycomb structure.
In a broad aspect, the present invention provides a method for producing a sealing element of the type specified at the beginning which combines simplicity of design and development with cost-effective, safe operation while avoiding the disadvantages of the state-of-the-art.
It is a particular object of the present invention to provide remedy to the above problems by the combination of the features cited in the independent claims, with further advantages becoming apparent from the sub-claims.
Accordingly, the present invention provides for bonding of the individual layers of the honeycomb structure and for bonding of the honeycomb structure to the backing by means of a brazing process in which pure aluminum or an aluminum alloy is used.